If you strongly suspect foul play in a tournament, you can approach the director after the event and present your evidence. Make sure you understand, however, that accusations and mere suggestions are an extremely serious business. Never accuse someone outright at the table of cheating. Doing so is considered one of the worst possible offenses in bridge. Even to simply imply someone is cheating is to start a heavy ball rolling that won't stop until you or the accused suffers profound embarrassment.
And remember: there is a big difference between those who are deliberately cheating to win an event, and those who don't know any better than to psych once too often or to hesitate at the wrong time. Both types of players should be reprimanded, but the difference in the weight of the accusations and the severity of the punishments is huge.
I must digress a little bit to tell a story. Many moons ago when I worked for OKbridge, we kicked a Flight C player off for cheating. At the same time, the monthly ACBL Bulletin came out featuring this player's picture on its cover as the year's top Flight C masterpoint winner. Personally I found this somewhat disgusting. A few months later I had the opportunity to play 2 boards against this guy in an ACBL event. As I sat down I asked him, "Hey, you're (so-and-so) on OKbridge, aren't you?" The first hand he flubs. "How do you like it so far?" I ask innocently. The second hand he flubs. As my partner and I leave the table I give him my business card and say, "If you need any help with your account...." =)